Broadband communications, radar and other imaging systems require the generation and transmission of radio frequency (“RF”) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many-applications at these high frequencies, a technique called “power combining” has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, “spatial power-combining” was proposed as a potential solution to these problems. Spatial power combining provides enhanced RF efficiency by coupling the components to beams or modes in free space rather than via transmission lines in corporate combining structures.
If components are combined using transmission line circuits there is an upper limit to the number of elements (and hence a limit to the power which can be generated) due to transmission line and combining structure losses that depend on the number of elements in a nonlinear relation or due to the accumulating complexity of the circuit. FIGS. 1a and 1b capture the concepts of corporate and spatial power combining, respectively. FIG. 1a shows a circuit for a corporate combiner of power amplifiers integrated on a planar geometry. It can be seen that as additional elements are added the lengths of transmission line and the number of nodal combining circuits increases. The losses in the added line and combining circuits accumulate. They reduce and eventually eliminate the advantages of the combined power. Hence, it becomes necessary to use a spatially combined architecture as shown in FIG. 1b. 
FIGS. 2a, 2b and 2c depict several spatially combined configurations to control the RF EM field of spacefed active arrays. FIG. 2a shows a quasi-optical system using lenses and polarizers to control the RF field. Similar configurations can use an open array with no lenses and may use phase control circuitry in each of the active antenna elements to provide beam control. FIG. 2b portrays an active array inside a waveguide, where the waveguide walls controls the EM field and defines its modal structure. FIG. 2c presents a waveguiding structure controlling EM field, but expanding to allow a larger number of amplifier elements to be combined.
Active arrays for space-fed spatial combining systems have been demonstrated in the two classic array topologies—tile and tray—shown in FIGS. 3a and 3b, respectively. In the case of the tile approach shown in FIG. 3a, two distinct design approaches have been developed, shown in FIGS. 4a and 4b. In the Rutledge “grid” array of FIG. 4a, active devices are integrated at the vertical and horizontal intersections of a metallic mesh. The vertical wires connect either the input circuits or the output circuits of the amplifiers, while the horizontal wires connect the other circuit. An incoming wave can thus be polarized to interact only with the amplifiers input circuits, while the outgoing wave will be orthogonally polarized. Polarizer grids used on either side of the grid array insure isolation between input and output circuits. In the grid array, the active elements are generally spaced much closer than a half wavelength. The entire length of the grid wires act as single antenna elements. In the active antenna array of FIG. 4b, separate antenna elements are integrated directly with active devices or a MMIC amplifier, with each element acting as an independent cell. The array acts as a periodic antenna array with the elements spaced at roughly half wavelength intervals. The EM wave is received on one side of the array, active devices can be placed on either or both sides of the array, and the array radiates on the other side. The antenna elements can be various combinations of patch and slot elements, with the possibility that in some configurations a common ground plane can isolate the input from the output. The tray approach, illustrated in FIG. 3b, uses a tray of end-fire antenna elements, with multiple trays stacked to provide a 2-dimensional array. The tray then acts to receive an input signal, to excite an electrical circuit that runs perpendicular to the plane of the antenna array, and to radiate from the other side of the trays.
Conventional spatial power-combiners do not efficiently combine power from a large number of active devices; do not obtain an equal share of power from each of the active device, irrespective of where the active device is located within the grid, and delivering the sum of the individual contributions to a load; and in high-power, high-frequency applications drawing heat away from spatial power combiners also remains a problem.
To overcome the problem of insufficient heat sinking a three-dimensional power combiner has been proposed as shown in FIGS. 5a and 5b. See W. A. Shiroma, B. L. Shaw and Z. B. Popovic, “Three-Dimensional Power Combiners,” 1994 IEEE MTT-S Int. Microwave Symp. Dig., pp. 831–834, May 1994; and W. A. Shiroma, B. L. Shaw and Z. B. Popovic, “A 100-Transistor Quadruple Grid Oscillator,” IEEE Microwave and guided Wave Lett., vol. 4, no. 10, pp. 350–351, October 1994. FIG. 5a depicts two planar oscillator grids placed back to back against a dielectric spacer and FIG. 5b depicts four planar oscillator grids separated by dielectric spacers.
However, there is still a need to more efficiently combine power from a large number of active devices. The present description addresses these problems by disclosing a three-dimensional power combiner to efficiently combine power from a large number of active devices (e.g. transistors, negative resistance diodes) in a series manner while obtaining an equal share of power from each active device, independent of where the active device is located along the series chain, and delivering the sum of the individual contributions to a load.